useless Rc<Rc<T>>, Rc<Box<T>>, Rc<&T>

Author: jpospychala

CHANGELOG.md

@@ -1195,7 +1195,6 @@ Released 2018-09-13
 [`bool_comparison`]: https://rust-lang.github.io/rust-clippy/master/index.html#bool_comparison
 [`borrow_interior_mutable_const`]: https://rust-lang.github.io/rust-clippy/master/index.html#borrow_interior_mutable_const
 [`borrowed_box`]: https://rust-lang.github.io/rust-clippy/master/index.html#borrowed_box
-[`box_borrows`]: https://rust-lang.github.io/rust-clippy/master/index.html#box_borrows
 [`box_vec`]: https://rust-lang.github.io/rust-clippy/master/index.html#box_vec
 [`boxed_local`]: https://rust-lang.github.io/rust-clippy/master/index.html#boxed_local
 [`builtin_type_shadow`]: https://rust-lang.github.io/rust-clippy/master/index.html#builtin_type_shadow
@@ -1434,6 +1433,7 @@ Released 2018-09-13
 [`range_plus_one`]: https://rust-lang.github.io/rust-clippy/master/index.html#range_plus_one
 [`range_step_by_zero`]: https://rust-lang.github.io/rust-clippy/master/index.html#range_step_by_zero
 [`range_zip_with_len`]: https://rust-lang.github.io/rust-clippy/master/index.html#range_zip_with_len
+[`redundant_allocation`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_allocation
 [`redundant_clone`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_clone
 [`redundant_closure`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure
 [`redundant_closure_call`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure_call

clippy_lints/src/lib.rs

@@ -795,7 +795,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &try_err::TRY_ERR,
         &types::ABSURD_EXTREME_COMPARISONS,
         &types::BORROWED_BOX,
-        &types::BOX_BORROWS,
         &types::BOX_VEC,
         &types::CAST_LOSSLESS,
         &types::CAST_POSSIBLE_TRUNCATION,
@@ -812,6 +811,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &types::LET_UNIT_VALUE,
         &types::LINKEDLIST,
         &types::OPTION_OPTION,
+        &types::REDUNDANT_ALLOCATION,
         &types::TYPE_COMPLEXITY,
         &types::UNIT_ARG,
         &types::UNIT_CMP,
@@ -1368,7 +1368,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&try_err::TRY_ERR),
         LintId::of(&types::ABSURD_EXTREME_COMPARISONS),
         LintId::of(&types::BORROWED_BOX),
-        LintId::of(&types::BOX_BORROWS),
         LintId::of(&types::BOX_VEC),
         LintId::of(&types::CAST_PTR_ALIGNMENT),
         LintId::of(&types::CAST_REF_TO_MUT),
@@ -1378,6 +1377,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&types::IMPLICIT_HASHER),
         LintId::of(&types::LET_UNIT_VALUE),
         LintId::of(&types::OPTION_OPTION),
+        LintId::of(&types::REDUNDANT_ALLOCATION),
         LintId::of(&types::TYPE_COMPLEXITY),
         LintId::of(&types::UNIT_ARG),
         LintId::of(&types::UNIT_CMP),
@@ -1662,8 +1662,8 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&redundant_clone::REDUNDANT_CLONE),
         LintId::of(&slow_vector_initialization::SLOW_VECTOR_INITIALIZATION),
         LintId::of(&trivially_copy_pass_by_ref::TRIVIALLY_COPY_PASS_BY_REF),
-        LintId::of(&types::BOX_BORROWS),
         LintId::of(&types::BOX_VEC),
+        LintId::of(&types::REDUNDANT_ALLOCATION),
         LintId::of(&vec::USELESS_VEC),
     ]);
 

clippy_lints/src/types.rs

@@ -172,33 +172,35 @@ declare_clippy_lint! {
 }
 
 declare_clippy_lint! {
-    /// **What it does:** Checks for use of `Box<&T>` anywhere in the code.
+    /// **What it does:** Checks for use of redundant allocations anywhere in the code.
     ///
-    /// **Why is this bad?** Any `Box<&T>` can also be a `&T`, which is more
-    /// general.
+    /// **Why is this bad?** Expressions such as `Rc<&T>`, `Rc<Rc<T>>`, `Rc<Box<T>>`, `Box<&T>`
+    /// add an unnecessary level of indirection.
     ///
     /// **Known problems:** None.
     ///
     /// **Example:**
     /// ```rust
-    /// fn foo(bar: Box<&usize>) {}
+    /// # use std::rc::Rc;
+    /// fn foo(bar: Rc<&usize>) {}
     /// ```
     ///
     /// Better:
     ///
     /// ```rust
+    /// # use std::rc::Rc;
     /// fn foo(bar: &usize) {}
     /// ```
-    pub BOX_BORROWS,
+    pub REDUNDANT_ALLOCATION,
     perf,
-    "a box of borrowed type"
+    "redundant allocation"
 }
 
 pub struct Types {
     vec_box_size_threshold: u64,
 }
 
-impl_lint_pass!(Types => [BOX_VEC, VEC_BOX, OPTION_OPTION, LINKEDLIST, BORROWED_BOX, BOX_BORROWS]);
+impl_lint_pass!(Types => [BOX_VEC, VEC_BOX, OPTION_OPTION, LINKEDLIST, BORROWED_BOX, REDUNDANT_ALLOCATION]);
 
 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Types {
     fn check_fn(
@@ -240,7 +242,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Types {
 }
 
 /// Checks if `qpath` has last segment with type parameter matching `path`
-fn match_type_parameter(cx: &LateContext<'_, '_>, qpath: &QPath<'_>, path: &[&str]) -> bool {
+fn match_type_parameter(cx: &LateContext<'_, '_>, qpath: &QPath<'_>, path: &[&str]) -> Option<Span> {
     let last = last_path_segment(qpath);
     if_chain! {
         if let Some(ref params) = last.args;
@@ -253,10 +255,27 @@ fn match_type_parameter(cx: &LateContext<'_, '_>, qpath: &QPath<'_>, path: &[&st
         if let Some(did) = qpath_res(cx, qpath, ty.hir_id).opt_def_id();
         if match_def_path(cx, did, path);
         then {
-            return true;
+            return Some(ty.span);
         }
     }
-    false
+    None
+}
+
+fn match_borrows_parameter(_cx: &LateContext<'_, '_>, qpath: &QPath<'_>) -> Option<Span> {
+    let last = last_path_segment(qpath);
+    if_chain! {
+        if let Some(ref params) = last.args;
+        if !params.parenthesized;
+        if let Some(ty) = params.args.iter().find_map(|arg| match arg {
+            GenericArg::Type(ty) => Some(ty),
+            _ => None,
+        });
+        if let TyKind::Rptr(..) = ty.kind;
+        then {
+            return Some(ty.span);
+        }
+    }
+    None
 }
 
 impl Types {
@@ -280,7 +299,6 @@ impl Types {
     /// The parameter `is_local` distinguishes the context of the type; types from
     /// local bindings should only be checked for the `BORROWED_BOX` lint.
     #[allow(clippy::too_many_lines)]
-    #[allow(clippy::cognitive_complexity)]
     fn check_ty(&mut self, cx: &LateContext<'_, '_>, hir_ty: &hir::Ty<'_>, is_local: bool) {
         if hir_ty.span.from_expansion() {
             return;
@@ -291,28 +309,19 @@ impl Types {
                 let res = qpath_res(cx, qpath, hir_id);
                 if let Some(def_id) = res.opt_def_id() {
                     if Some(def_id) == cx.tcx.lang_items().owned_box() {
-                        if_chain! {
-                            let last = last_path_segment(qpath);
-                            if let Some(ref params) = last.args;
-                            if !params.parenthesized;
-                            if let Some(ty) = params.args.iter().find_map(|arg| match arg {
-                                GenericArg::Type(ty) => Some(ty),
-                                _ => None,
-                            });
-                            if let TyKind::Rptr(..) = ty.kind;
-                            let ty_ty = hir_ty_to_ty(cx.tcx, ty);
-                            then {
-                                span_lint_and_help(
-                                    cx,
-                                    BOX_BORROWS,
-                                    hir_ty.span,
-                                    "usage of `Box<&T>`",
-                                    format!("try `{}`", ty_ty).as_str(),
-                                );
-                                return; // don't recurse into the type
-                            }
+                        if let Some(span) = match_borrows_parameter(cx, qpath) {
+                            span_lint_and_sugg(
+                                cx,
+                                REDUNDANT_ALLOCATION,
+                                hir_ty.span,
+                                "usage of `Box<&T>`",
+                                "try",
+                                snippet(cx, span, "..").to_string(),
+                                Applicability::MachineApplicable,
+                            );
+                            return; // don't recurse into the type
                         }
-                        if match_type_parameter(cx, qpath, &paths::VEC) {
+                        if let Some(_) = match_type_parameter(cx, qpath, &paths::VEC) {
                             span_lint_and_help(
                                 cx,
                                 BOX_VEC,
@@ -322,6 +331,43 @@ impl Types {
                             );
                             return; // don't recurse into the type
                         }
+                    } else if Some(def_id) == cx.tcx.lang_items().rc() {
+                        if let Some(span) = match_type_parameter(cx, qpath, &paths::RC) {
+                            span_lint_and_sugg(
+                                cx,
+                                REDUNDANT_ALLOCATION,
+                                hir_ty.span,
+                                "usage of `Rc<Rc<T>>`",
+                                "try",
+                                snippet(cx, span, "..").to_string(),
+                                Applicability::MachineApplicable,
+                            );
+                            return; // don't recurse into the type
+                        }
+                        if let Some(span) = match_type_parameter(cx, qpath, &paths::BOX) {
+                            span_lint_and_sugg(
+                                cx,
+                                REDUNDANT_ALLOCATION,
+                                hir_ty.span,
+                                "usage of `Rc<Box<T>>`",
+                                "try",
+                                snippet(cx, span, "..").to_string(),
+                                Applicability::MachineApplicable,
+                            );
+                            return; // don't recurse into the type
+                        }
+                        if let Some(span) = match_borrows_parameter(cx, qpath) {
+                            span_lint_and_sugg(
+                                cx,
+                                REDUNDANT_ALLOCATION,
+                                hir_ty.span,
+                                "usage of `Rc<&T>`",
+                                "try",
+                                snippet(cx, span, "..").to_string(),
+                                Applicability::MachineApplicable,
+                            );
+                            return; // don't recurse into the type
+                        }
                     } else if cx.tcx.is_diagnostic_item(Symbol::intern("vec_type"), def_id) {
                         if_chain! {
                             // Get the _ part of Vec<_>
@@ -359,7 +405,7 @@ impl Types {
                             }
                         }
                     } else if match_def_path(cx, def_id, &paths::OPTION) {
-                        if match_type_parameter(cx, qpath, &paths::OPTION) {
+                        if let Some(_) = match_type_parameter(cx, qpath, &paths::OPTION) {
                             span_lint(
                                 cx,
                                 OPTION_OPTION,

clippy_lints/src/utils/paths.rs

@@ -10,6 +10,7 @@ pub const BEGIN_PANIC: [&str; 3] = ["std", "panicking", "begin_panic"];
 pub const BEGIN_PANIC_FMT: [&str; 3] = ["std", "panicking", "begin_panic_fmt"];
 pub const BINARY_HEAP: [&str; 4] = ["alloc", "collections", "binary_heap", "BinaryHeap"];
 pub const BORROW_TRAIT: [&str; 3] = ["core", "borrow", "Borrow"];
+pub const BOX: [&str; 3] = ["alloc", "boxed", "Box"];
 pub const BTREEMAP: [&str; 5] = ["alloc", "collections", "btree", "map", "BTreeMap"];
 pub const BTREEMAP_ENTRY: [&str; 5] = ["alloc", "collections", "btree", "map", "Entry"];
 pub const BTREESET: [&str; 5] = ["alloc", "collections", "btree", "set", "BTreeSet"];

src/lintlist/mod.rs

@@ -101,13 +101,6 @@ pub static ref ALL_LINTS: Vec<Lint> = vec![
         deprecation: None,
         module: "types",
     },
-    Lint {
-        name: "box_borrows",
-        group: "perf",
-        desc: "a box of borrowed type",
-        deprecation: None,
-        module: "types",
-    },
     Lint {
         name: "box_vec",
         group: "perf",
@@ -1732,6 +1725,13 @@ pub static ref ALL_LINTS: Vec<Lint> = vec![
         deprecation: None,
         module: "ranges",
     },
+    Lint {
+        name: "redundant_allocation",
+        group: "perf",
+        desc: "redundant allocation",
+        deprecation: None,
+        module: "types",
+    },
     Lint {
         name: "redundant_clone",
         group: "perf",

tests/ui/box_borrows.rs

@@ -1,6 +1,6 @@
 // run-rustfix
 #![feature(never_type)]
-#![allow(unused_mut)]
+#![allow(unused_mut, clippy::redundant_allocation)]
 #![warn(clippy::must_use_candidate)]
 use std::rc::Rc;
 use std::sync::atomic::{AtomicBool, Ordering};

tests/ui/box_borrows.stderr

@@ -1,6 +1,6 @@
 // run-rustfix
 #![feature(never_type)]
-#![allow(unused_mut)]
+#![allow(unused_mut, clippy::redundant_allocation)]
 #![warn(clippy::must_use_candidate)]
 use std::rc::Rc;
 use std::sync::atomic::{AtomicBool, Ordering};